In the electronics art, it is frequently necessary to apply a resist pattern to the surface of a conductive material, as for example, in the fabrication of printed circuit boards.
A printed circuit board, also known as a printed wiring board, is most commonly fabricated from a base of a suitable insulating material in the form of a rigid or flexible sheet coated with a layer of conductive material (e.g. metal). To fabricate the printed circuit, it is customary to form a resist pattern on the surface of the conductive material in the pattern prescribed by the circuit design, and to then place the board in an appropriate etching solution to remove metal from the areas not covered by the resist pattern. For example, in the case of a rigid phenolic board coated with copper foil: (a) a resist image in the pattern of the desired circuit is first formed as a coating on the surface of the copper foil; (b) the coated surface of the copper foil is then immersed or otherwise contacted with an acid or alkaline etch solution to remove the copper foil layer from those areas of the board not covered by the resist pattern. During the contact with the etch solution, the copper foil layer in the areas covered by the resist pattern is not removed. As a result, after the resist coating is removed from the board, there remains a printed circuit of copper foil in the desired pattern on the phenolic support.
In fabricating printed circuit boards using a resist material, the resist pattern must be accurately formed on the surface of the conductive layer. Any substantial deviation from the prescribed pattern can result in a printed circuit board that does not function in the proper way.
In recent years, at least two factors have combined to make the fabrication of printed circuits in accordance with their specifications an increasingly difficult task. First, the circuits have become more complex and, second, the density of such circuits has increased. These factors have resulted in an ever-increasing need to form resist images of high resolution and precision in connection with the fabrication of printed circuit boards.
Substantial efforts have been devoted in the printed circuit art to develop techniques for applying resist patterns with precision in a commercially practicable manner. Two techniques that have been developed and that have achieved the most commercial success are the screen printing method and the photopolymer resist method.
In the screen printing method, the traditional techniques used in silk-screen printing have been applied to printing an ink resist pattern on a conductor coated on a base of insulator material. Screen printing is low in ink cost, but is disadvantageous in that it requires the costly set-up of a master. Also, the screen printing method has generally only been implemented as a flat-bed process, which requires extensive operator interaction to maintain registration and correct ink viscosity. Additional disadvantages of the screen printing method include limited edge definition of the printed image, relatively poor resolution of the printed image and the necessity for post curing of the printed image. As a result of the foregoing disadvantages, screen printing is typically used only for large production runs of low to medium density printed circuits.
In the photopolymer resist method, the entire metal surface of a metal coated board is first covered with a layer of a suitable photopolymer. The photopolymer may be applied as a liquid which is then dried, or it may be applied in dry-film form. The photopolymer is then exposed to actinic radiation through a photographic mask held against the photopolymer layer and developed to form the desired resist pattern. The photographic mask must be replaced after a certain number of exposures, typically about 20, due to scratching of the mask. This method provides a higher quality resist image than does the screen printing method, but has disadvantages, including the high cost of the necessary materials, the need to replace the mask at regular intervals, and the multiple, time consuming steps necessary to obtain the resist image.
Electrostatic imaging techniques using dry toner have been proposed to overcome the disadvantages of the methods described above. Such techniques, however, have not met with substantial success, at least in part, because of problems in forming high resolution resist patterns on conductor surfaces. For example, such electrostatic imaging techniques have been plagued by low resolution, insufficient resist in the image areas and pin-holes in the resist image.
The problems with electrostatic imaging techniques described in the preceding paragraph have been due, at least in part, to the unavailability of liquid electrostatic toners possessing satisfactory edge-forming, etch resist and wet-transfer characteristics. For example, known liquid toners are capable of forming high resolution images on an absorbent substrate, such as paper, but such images generally may not be transferred to another substrate since they tend to be absorbed into the first substrate. Moreover, even if they were capable of transfer from an absorbent substrate, images produced using the known liquid toners, to our knowledge, are not sufficiently resistant to etching solutions to be useful in the fabrication of printed circuits. Also, attempts have been made in conventional electrophotographic processes to transfer a liquid toned image from a photoconductive surface onto a nonabsorbent substrate such as plastic. Such attempts have been characterized by problems with smearing and running of the transferred image.
Attempts to transfer a dry toner image directly from a photoconductive surface to a conductor in conventional electrophotography have likewise been met with problems, particularly in achieving good resolution of the transferred image. For example, one source of the problems with these attempts has been the tendency of the toner particles employed to take on the same charge as the conductor and, consequently, being repelled by the conductor surface. Attempts to use a thin film of insulating liquid to facilitate the transfer of the toner image to the surface of a conductor similarly have had problems because the toner has tended to be dispersed in the insulating liquid. See U.S. Pat. No. 4,444,858 at col. 2, lines 4-9 and col. 4, lines 61-65 concerning the foregoing problems.